1. Field of the Invention
The present invention provides a method for manufacturing foamed-thermoplastic films for use in blown-film applications such as bags.
2. Description of the Related Art
The terms “thermoplastic film” or “film,” as used herein, mean thin flexible materials made from thermoplastic resins. Films usually have a thickness of not greater than 0.25 millimeters (mm). These materials are used particularly in blown-film applications such as consumer trash-bags, grocery bags, produce bags, pallet wrap, food wrap, liners and the like. Films should be distinguished from other more rigid articles including sheets, tubes, boards and plates.
In the thermoplastic film industry, there is a need to produce thicker thermoplastic films at a cost savings to the film manufacturer. Negative consumer perceptions about thinner films currently being manufactured, recent high resin costs, and very low industry profit margins have created this need. This is especially true in the consumer trash-bag industry where intense competition has eroded already thin profit margins, and raw material costs are a very large component of production costs. Within the past several years, trash-bag manufacturers have been producing thinner films as a result of the advent of high-strength polymer resins. These high-strength polymer resins have enabled manufacturers to down-gauge the thickness of the film and still maintain the same toughness and strength of the film. Although this ability to down-gauge has resulted in a small cost savings to the manufacturer, consumers have not been satisfied with these new thinner films, especially when used in consumer trash-bags, as they perceive the bags to be too flimsy, too thin, and difficult to open.
If a trash-bag manufacturer were to address these consumer concerns by manufacturing bags made from thicker thermoplastic films using known processes, he must use more thermoplastic resin, which results in a significant cost increase to himself that he cannot pass on to the consumer. Further, the use of additional resin results in an increased amount of nonbiodegradeable waste that must be disposed of in places such as landfills. This is particularly bothersome given that oil, the raw material for plastics, is a nonrenewable resource. Increasing the thickness of the film used to make the bag to satisfy consumers, while retaining an ability to reduce the amount of thermoplastic resin used to create the film, presents a significant advantage to a manufacturer—not only in terms of cost savings, but also in terms of consumer satisfaction with his product. It would therefore be desirable if an improved method were available for producing thicker thermoplastic films that are more satisfying to consumers without using additional thermoplastic resin so that the consumer trash-bag manufacturer would be able to retain his cost-advantage that he has obtained through down-gauging. Additionally, it would be desirable if an improved method were available for producing thinner yet more satisfying films using even less thermoplastic material than is currently being used. It would also be desirable if such films retained adequate toughness and strength wherein the film could be used effectively in consumer trash-bags.
Although it is known that thicker films can be produced via a foaming process, conventional foamed-thermoplastic films do not have sufficient physical characteristics to be utilized in consumer trash-bags due to their reduced mechanical, impact and tensile strength. For instance, Francis C. Shea's article “Processing Foam Film,”(1977) discloses that tensile strength, impact strength, and elongation decrease as the density of a foamed film decreases. A foamed film's density is inherently reduced by the foaming process; therefore, its physical properties are necessarily adversely affected. Consequently, it is not practical to use conventional foamed-thermoplastic films in blown-film applications like trash-bags. Furthermore, heretofore known processes for foaming thermoplastic films produce gaseous cells of uneven structure, which give surface irregularities and poor appearance to the resulting film, thereby yielding products of limited commercial value.
Therefore, there is an unaddressed need for thicker thermoplastic films that can be used effectively in commodity blown-film applications that require toughness and strength such as consumer trash-bags. Such films should have a pleasing appearance and feel to the consumer, and should furnish the manufacturer with the ability to reduce the amount of thermoplastic resin used to produce thinner films.